1. Field of the Invention
The present invention particularly relates to a switching mechanism of the magnetization state of a magnetoresistive element serving as a data storage medium of a magnetic random access memory.
2. Description of the Related Art
Various types of solid-state magnetic memories have been proposed conventionally. In recent years, magnetic random access memories using magnetic memory elements which exhibit a giant magnetoresistive effect have been proposed. As a result, the dominating magnetic memory elements presently use a ferromagnetic tunnel junction.
A ferromagnetic tunnel junction has a layered structure of, e.g., a ferromagnetic layer/insulating layer (tunnel barrier layer)/ferromagnetic layer. When a voltage is applied to the insulating layer, a tunnel current flows to the insulating layer. In this case, the junction resistance value of the ferromagnetic tunnel junction (the tunnel conductance of the insulating layer) changes in accordance with the cosine of the relative angle between the magnetizations of the two ferromagnetic layers.
The junction resistance value is minimum when the magnetizations of the two ferromagnetic layers are set in the same direction (parallel state) or maximum when the magnetizations have reverse directions (anti-parallel state).
This phenomenon is called a tunnel magnetoresistive (TMR) effect. Recent reports have revealed, e.g., that the change rate (MR ratio) of the resistance value of a magnetic tunnel junction (MTJ) element by the TMR effect is 49.7% at room temperature.
In a magnetic memory element having a ferromagnetic tunnel junction, one of two ferromagnetic layers is a reference layer (pinned layer) with a fixed magnetization state, and the other is a storage layer (free layer) whose magnetization state changes in accordance with data. When the magnetizations of the reference layer and storage layer are parallel, the state is defined as, e.g., “0”, and the anti-parallel state is defined as “1”.
Data is written by, e.g., applying, to the magnetic memory element, a magnetic field generated by a write current supplied to a write line and inverting the magnetization direction of the storage layer of the magnetic memory element. Data is read out by supplying a read current to the ferromagnetic tunnel junction of the magnetic memory element and detecting a change in resistance of the ferromagnetic tunnel junction caused by the TMR effect.
A magnetic memory is formed by arranging such magnetic memory elements in an array. In the actual structure, one switching transistor is connected to one magnetic memory element to enable random access to the magnetic memory elements, like, e.g., a dynamic random access memory (DRAM).
There is also proposed another technique in which a magnetic memory element formed by combining a diode and a ferromagnetic tunnel junction is arranged at the intersection between a word line and a bit line.
From the viewpoint of integration of magnetic memory elements with ferromagnetic tunnel junctions, the cell size must be small. Hence, the size of the ferromagnetic layer of the magnetic memory element inevitably decreases. Generally, the coercive force of a ferromagnetic layer increases in inverse proportion to its size.
The coercive force can be used as a measure of the magnitude of a switching field necessary for inverting magnetization. An increase in coercive force means an increase in magnitude of the switching field of the magnetic memory element.
Hence, if the size of the ferromagnetic layer is reduced by micropatterning the magnetic memory element, a large write current is necessary in the data write. This leads to an undesirable increase in power consumption.
For practical use of a bulk magnetic memory, it is therefore indispensable to simultaneously reduce the size of a magnetic memory element and the coercive force of a ferromagnetic layer used in it.
A solid-state magnetic memory must stably store data because it operates as a nonvolatile memory. There is a parameter called a thermal fluctuation constant, which can be used as a criterion for determining whether data can be stored stably for a long time. As is generally known, the thermal fluctuation constant is proportional to the volume and coercive force of the ferromagnetic layer.
When the coercive force of the ferromagnetic layer is decreased to reduce power consumption, the thermal stability degrades, and it becomes impossible to store data for a long time. That is, it is also an important challenge to realize practical use of a bulk magnetic memory having a magnetic memory element which has high thermal stability and is capable of continuously storing data for a long time.
When a magnetic memory is formed by magnetic memory elements, the shape of the magnetic memory element is often set to a rectangle.
However, as the size of the magnetic memory element decreases, a special magnetic domain called an edge domain is generated at each short-side end of the ferromagnetic layer included in the magnetic memory element, as is known.
The edge domain indicates a magnetic domain at a short-side end of a rectangular ferromagnetic layer, where magnetization rotates in a spiral shape along the short sides. An edge domain phenomenon indicates a phenomenon that the magnetization of a rectangular ferromagnetic layer rotates in a spiral shape along the short sides at the short-side ends.
The edge domain phenomenon occurs when the demagnetization energy at the short-side ends of the rectangular ferromagnetic layer is reduced. FIG. 105 shows an example of such a magnetic structure (magnetic domain). At the central portion of the magnetization region, magnetization occurs in the direction along the magnetic anisotropy, i.e., in the direction along the long sides. At the two ends, however, magnetization occurs in a direction different from the central portion, i.e., in the direction along the short sides.
Flux reversal in the magnetic memory element will be examined. When a magnetic field is applied to the rectangular ferromagnetic layer, the edge domain of the ferromagnetic layer gradually becomes large. Magnetizations in the edge domains at the two short-side ends of the ferromagnetic layer are parallel or anti-parallel to each other.
When the edge domains are directed in the same direction (parallel), a domain wall of 360° is formed, and the coercive force increases.
To solve this problem, a technique of using an elliptical ferromagnetic layer as the storage layer has been proposed (e.g., reference 1).
The edge domain is very sensitive to the shape of the ferromagnetic layer. When the storage layer has an elliptical shape, occurrence of an edge domain can be prevented, and a single domain can be implemented. The magnetization direction can uniformly be reversed throughout the ferromagnetic layer. Hence, the switching field (reversal field) can be reduced.
There is also proposed a technique of using, as the storage layer, a ferromagnetic layer of a shape such as a parallelogram shape whose short and long sides do not meet at right angles (e.g., reference 2).
In this case, an edge domain still exists though it occupies not so large area as in a rectangular layer. Since no complex small domain is generated in the process of flux reversal, flux reversal can occur almost uniformly. As a consequence, the switching field (reversal field) can be reduced.
Still another structure has been proposed in which a plurality of basic structures each including a ferromagnetic layer/nonmagnetic layer/ferromagnetic layer are stacked while keeping the rectangular shape of the ferromagnetic layer unchanged (e.g., reference 3).
In this case, when the two ferromagnetic layers stacked have different magnetic moments or thicknesses, magnetizations of the ferromagnetic layers can be directed in reverse directions by antiferromagnetic coupling. For this reason, effectively, the magnetizations cancel each other. The whole storage layer becomes equivalent to a ferromagnetic layer having small magnetization in the direction of axis of easy magnetization.
When a magnetic field is applied in a direction reverse to the direction of small magnetization in the direction of axis of easy magnetization of the storage layer, the magnetization of the storage layer reverses while maintaining antiferromagnetic coupling. Since the lines of magnetic force are closed, the influence of the demagnetizing field is small. The switching field of the storage layer is determined by the coercive force of the two ferromagnetic layers. That is, the switching field is small, and flux reversal easily occurs.
As described above, reducing the magnetic field (switching field) necessary for flux reversal in the storage layer and improving the thermal stability are indispensable factors for the magnetic memory. To implement them, various kinds of shapes and also a multilayered structure of ferromagnetic layers which are ferromagnetically coupled have been proposed until now in regard to the magnetic memory element.
However, in the ferromagnetic layer of a small magnetic memory element used in a highly integrated magnetic memory, when the length of the short side is, e.g., several μm or submicron or less, a magnetic structure (edge domain) different from that of the central portion is generated because of the influence of the demagnetizing field at the ends of the magnetization region.
In a small magnetic memory element used in a highly integrated magnetic memory, the influence of an edge domain generated at its ends is large, and the change in magnetic structure (magnetization pattern) in flux reversal is complex. As a result, the coercive force increases, and the switching field increases.
To prevent such a complex change in magnetic structure as much as possible, the edge domain is fixed (e.g., reference 4).
With this method, the behavior at the time of flux reversal can be controlled. However, the switching field cannot be reduced substantially. In addition, to fix the edge domain, another structure needs to be added, and it is inappropriate for increasing the density of magnetic memory elements.
References of the prior arts related to the present invention are as follows:
Reference 1: U.S. Pat. No. 5,757,695
Reference 2: Jpn. Pat. Appln. KOKAI Publication No. 11-273337
Reference 3: U.S. Pat. No. 5,953,248
Reference 4: U.S. Pat. No. 5,748,524
Reference 5: Jpn. Pat. Appln. KOKAI Publication No. 2004-128067
Reference 6: Jpn. Pat. Appln. KOKAI Publication No. 2004-146614
Reference 7: U.S. Pat. No. 5,640,343
Reference 8: U.S. Pat. No. 5,650,958